<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>4_getSnapshotBeforeUpdate的使用场景</title>
  <style>
    .list{
      width: 200px;
      height: 150px;
      background-color: skyblue;
      overflow: auto;
    }
    .news{
      height: 30px;
    }
  </style>
</head>
<body>
<div id="test"></div>
<!--引入 react核心库-->
<script type="text/javascript" src="../js/17.0.1/react.development.js"></script>
<!--引入 react-dom 用于支持 react 操作 DOM-->
<script type="text/javascript" src="../js/17.0.1/react-dom.development.js"></script>
<!--引入 babel 用于将 jsx 转为 js-->
<script type="text/javascript" src="../js/17.0.1/babel.min.js"></script>

<script type="text/babel">
  class NewsList extends React.Component{

    state = {newsArr:[]}

    componentDidMount(){
      setInterval(()=>{
        // 获取原状态
        const {newsArr}=this.state
        // 模拟一条新闻
        const news = '新闻'+(newsArr.length+1)
        // 更新状态
        this.setState({newsArr:[news,...newsArr]})
      },1000)
    }

    getSnapshotBeforeUpdate(){
      return this.refs.list.scrollHeight
    }

    componentDidUpdate(preProps,preState,height){
      this.refs.list.scrollTop += this.refs.list.scrollHeight - height
    }

    render(){
      return(
          <div>
            <div className="list" ref="list">
              {
                this.state.newsArr.map((n,index)=>{
                  return <div key={index} className="news">{n}</div>
                })
              }
            </div>
          </div>
      )
    }
  }

  ReactDOM.render(<NewsList/>,document.getElementById('test'))
</script>
</body>
</html>
